Reforming, such as steam reforming, is a catalytic reaction in which a mixture of steam and hydrocarbons are exposed to a catalyst under specified conditions to produce a mixture of carbon oxides and hydrogen. This resultant mixture is commonly known as syngas.
Syngas can then be further processed using conventional methods into several products, such as hydrogen, methanol, and ammonia, which have many industrial and commercial uses.
The basic chemical reactions that result in the production of syngas, and the further processing of syngas into other products, have been well known for many years. Indeed, reforming has been long and widely practiced on a large, industrial scale.
Industrial steam reformers are generally of a tubular construction, in which one or more (usually at least several) large elongate metal tubes are packed with the reforming catalyst. The reformate mixture is then caused to flow through the tubes, and hence coming into contact with the catalyst. Because the catalytic reaction in these processes requires added heat, some means to raise the reformate/catalyst system to the requisite temperature must be provided. In the typical tubular industrial reforming systems, the outer surface of the tubes is usually heated by a combination of convective and radiant heat transfer from some form of external combustion.
Thus, the successful operation of a tubular reformer relies on maintaining a somewhat delicate balance between the endothermic reforming reaction within the tubes and the heat transfer to the tubes from the external combustion process. Also, the provision of heat to the tubes must be uniformly controlled, as on one hand excessive heat input at any point might lead to hot spots on the tubes that could in turn lead to tube failure or “coking” of the hydrocarbon (which adversely affects the reforming process); or, on the other hand, inadequate heat input at some or all of the tube, which would result in low hydrocarbon conversion. Another important variable for these traditional reformers is pressure, and more specifically, pressure drops at various stages of the reforming process. Therefore, the operation of these large-scale tubular reformers has typically involved a rather complex control system comprising multiple control loops.
While these known large scale processes and equipment in which the processes have been carried out have generally worked well and cost-effectively, they have not proven effective for smaller scale reforming. Among other things, the costs to manufacture, install, maintain and operate these types of reformers on a smaller scale have been prohibitive. Therefore, smaller industrial and commercial applications have not been well-utilized.
In the past, smaller users of hydrogen, ammonia and methanol could not cost-effectively provide for on-site production of those products. Rather, such users would be required either to utilize other products which might not be optimal, or to have product delivered. For example, smaller users of hydrogen typically utilize the ubiquitous metal hydrogen-containing cylinders that are delivered by truck to the user's location. While this is a reliable source of the desired product, it is becoming less desirable as the price of vehicle fuel has dramatically increased, which in turn has driven up the cost of delivery and hence the cost of the product to the user. Also, many smaller users would prefer to have on-site production capability that will not only reduce cost over the longer run, but will also provide a captive source such that the user is not vulnerable to the vagaries of price, availability and delivery factors that are beyond the user's control.
Also, as the cost of traditional sources of energy has increased, the desire for a cost-effective alternative has risen.
Therefore, there has long existed a need for the ability to produce syngas on a smaller scale than has been cost-effectively possible with the heretofore used tubular systems, and that need continues to escalate. Providing such a solution, however, is neither easy nor obvious. Not only must such a smaller-scale operation be capable of cost-effective production of syngas in smaller quantities, but the operation of the system must be relatively simple, as the on-site production by most users will be a side-line operation, such that an overly complex system (with many control loops and sub-systems) that would require extensive training and expertise, and constant oversight during operation, would indirectly and adversely affect the cost efficiencies that on-site production is to achieve.
In other words, the preferred system will utilize passive controls at as many of the system's component aspects as possible, rather than requiring extensive control mechanisms and/or operator oversight. Because, however, the reforming process inherently includes multiple constituent products being subjected to high heat and potentially high pressure, whereas various aspects of the overall system are most cost-effectively conducted at relatively low pressures (for example, avoiding fuel and combustion air compression), providing a system that can be more than less passively controlled has proven to be a challenge.
Additionally, because a major component of any reforming operation is the provision of heat, cost effectiveness on a smaller scale requires that the efficiency of the process in terms of output per applied heat be high. This efficiency has proven difficult to achieve in smaller scale reformers, and would require that the heat generated by the system be effectively captured and reused.
Also, the on-site user will typically be utilizing the output of the system for personal consumption and use (as opposed to a larger scale production in which the production of syngas and one of the further refined products is itself the mainstay of the business). Therefore, while the larger producer will typically be operating the system at near maximum capacity for long periods of time, or even more or less continuously, it is unlikely that the smaller user will want the reforming process to be operating at maximum output continuously, or even most of the time. Rather, the smaller user may want to increase production as needed, and then reduce production during periods of low demand.
Yet, because traditional reformers require the maintenance of a balance of various factors for optimal (actually, even successful) operation, repeatedly turning the system completely off, to be restarted when the need for more product later arises, is not preferred. Therefore, the preferred smaller-scale reformer would provide the operator not only with the ability to turn production up and down as required by the user's on-site needs, but also allow the operator to reduce production to a very low level easily and without major control issues, and without having to completely turn the process off, only to have to re-start the process from standstill condition at some later time.
The preferred smaller-scale reformer should also avoid or minimize upkeep and repair. For example, a problem often encountered in any reforming operation is a condition or reaction called “metal dusting,” which causes the exposed metal in the reforming system to flake off metal particles, and if sufficiently severe, ultimately to fail altogether. Because it is difficult to exclude in any reforming process the conditions that will be conducive to metal dusting, it is particularly important in a reforming apparatus that the process control and localize metal dusting to the extent possible.
Another problem that can be encountered even in the smaller scale systems is coking. Coking occurs when the reactants within the system are subjected to a confluence of pressure and temperature conditions. Thus, the smaller systems must also address and avoid this potential problem.
A significant cost item in any reforming process is the catalyst utilized, both in chemical type of catalyst, and also in the form in which the catalyst is delivered and utilized in the system. For example, in the large scale reformers, large catalyst particles are usually employed for various reasons, some having to do with pressure and pressure drop issues in the system, and others having to do with the desired efficiency of output of the syngas, as the type and form of catalyst plays and integral role in the overall efficiency of production.
In a reformer and reforming process primarily intended for the smaller user, however, utilizing exotic, more expensive catalysts will not be preferred, nor will the larger particles be particularly well-adapted for use in a smaller-size apparatus. Accordingly, the preferred system will be able to use the more commonly available, off-the-shelf catalysts that are carried on a more varied and smaller size medium than is traditionally used in the large scale reformers. The preferred system will also use as a fuel source natural gas that is widely available.
As result of the drawbacks in the prior art reformers and the issues that arise when those systems are considered as a starting point for scaling down for lower production users, some consideration has been given to the possible development of an alternative to the tubular reformers; that is, to the use of so-called printed circuit heat exchanger (“PCHE”) cores and to the deposition of thin layers of reforming catalyst into channels of plates that form the cores. The PCHE cores currently are used in heat exchangers, and they are constructed by etching channels having required forms and profiles into one surface of individual plates which are then stacked and diffusion bonded to form cores having dimensions required for specific applications.
However, whilst this alternative approach does indicate some merit, several problems are foreseen, including the following:                Difficulties in obtaining adhesion of catalyst to the metal (plate) substrate,        Limited catalyst life,        Difficulties in replacing the catalyst, and        Coupling of heat transfer and catalyst areas, this requiring very high-activity catalyst if over-investment in heat exchange surface is to be avoided.        
A partial solution to these problems is revealed in United States Patent Publication US2002/0018739 A1, dated 14 Feb. 2002, which (without constituting common general knowledge or prior art) discloses a chemical reactor having a PCHE-type core. The core is constructed with alternating heat exchange and catalyst-containing zones that together form a passageway for a reactant. Each of the heat exchange zones is formed from stacked diffusion bonded plates, with some of the plates providing channels for (externally heated or cooled) heat exchange fluid and others of the plates providing orthogonally directed channels to carry the reactant from one catalyst-containing zone to the next such zone.
However, this arrangement has not proven sufficiently cost-effective and sufficiently simple to use as to provide a solution for the smaller-user.
Thus, there remains a long felt, and until now, unfulfilled, need in the art for a reforming process and apparatus that can be utilized by the smaller user and that will accomplish the multiple goals of being sufficiently cost efficient, easy to operate, monitor, maintain and control, and still provide high quality and quantity of syngas that will allow the smaller user to produce, on site, hydrogen, methanol and/or ammonia at the times required on-site.